@melkoutch Well, this is not a radio (sx127x) effect. If you look at adafruit lib (link in my previous post) it seem that this lib insert a 4 bytes header (line 649) before send a packet and line 697 you can se that you have to set with_header=False when you call rfm9x receive function to remove this 4 bytes header from your received data.

It seem there no similar way for send function so header will be always send by rfm9x library.
0xFF 0xFF 0x00 0x00 is broadcast header for RadioHead library, i suggest you to find and read this library specification if you plan to use it.

In particular for your usage, they don't set a value for lora sync word, so sx1276 use it's reset value 0x12 (private lora), but it's seem that pycom init value is 0x34 (public lora aka lorawan).
I suggest you to test on your pycom device with

@melkoutch I still feel uncomfortable about it, since the test failed initially, and worked later on when returning to the initial set-up. So it still has the smell of a non-initialized-variable bug somewhere.

@melkoutch No clue why it does not work. This is similar to the simple LoPy-Lopy ping-pong example from the docs and library, which I tested now again between two LoPy4 devices, AND IT FAILS. It used to work.
Edit 1: There is a RF transmission, but no data received.
Edit 2: Just taking the example without setting the frequency works. The default is 868Mhz It has to be noted that when changing the parameters a HARD reset is required.
Edit 3: Again tested, works now also with changed frequency ar 863MHz. One thing I did was to mis-align the antennas, because devices are very close on my desk. With one antenna vertical, and the other horizontal I have an rssi of about -50 dB, which is fine.